1. Field of the Invention
This invention is related to a light measuring apparatus and a method of using the device. It is used to measure various photometric quantities of the light emanating from a distant source of light. In this device the field of selected light is more accurately measured than in other devices known to the inventors.
Generally a source of light, particularly an exterior source of light, is configured and positioned to cast light in a desired pattern. Often the source of light will indiscriminately emit light such that the some of the emitted light is directed beyond the zone to be illuminated. Light extending beyond the intended zone of the lighting system is “spillover.” When spillover extends beyond property lines it is known as “trespass light” and may be objectionable for various reasons to people subjected to intrusive trespass light.
The device presented in this disclosure measures the light emitted from a distant source of light. The distant source of light may be the source of spillover light, trespass light and a source of glare. The measurement of spillover and trespass light is the target of the light measuring apparatus presented herein. The measurement of the trespass light will be used to determine the acceptability of the source of light configuration, that is, whether or not the trespass light or glare is acceptable or unacceptable to a viewer in a particular situation.
A telescopic light-sensing meter used to measure received levels of distant sources of light is disclosed. The light-sensing device is connected to, in one embodiment, an onboard processing unit that will process information related to light measurements being performed by the meter. The processor will also receive information from an integrated distance-measuring device. Data from the distance measuring device and the light measurements collected in the apparatus are processed and a finite number representing a measurement of light intensity emitted by a source of light is displayed to the equipment user.
One aspect of this invention is that the light measurement path through the light meter is also the path taken by reflected laser pulses emanating from the distance measuring device of the light measuring apparatus and reflected back from a target source of light. Thus a single light path through the light measuring apparatus is used for the distance-measuring device and for light measurement.
2. Description of the State of the Art
Outdoor light measuring meters are available to measure illuminance values in lux or footcandles. These state of the art light meters measure only the illuminance of the area on which the light falls. These are simple portable meters with a light sensitive cell that is positioned on the surface of interest and generates a signal to an appropriate display, which then indicates the illuminance level of the surface being measured. This type of meter is effective in measuring illuminance on vertical surfaces, such as a vertical window of a residence. The meter may also be used to measure illuminance on a horizontal surface, such as on a floor of a building. This type of light measuring meter and the above method does not meet today's need of measuring spillover or trespass lighting that needs to be measured in terms of light intensity.
Other light measuring meters referred to as telephotometers, are available. Telephotometers may use a lens to focus the image of a source of light of interest onto a light sensitive cell. Frequently the focusing optics is similar to those used in a telescope and can be used to view light from a distant source. The quantity of light measured by a telephotometer is the luminance of the area at which the telephotometer is aimed, luminance being the intensity of light per unit area of the source of light area being measured. Luminance measurements are usually stated as footlamberts or candelas per square meter.
Other terms pertinent to light measuring devices are terms such as “luminous intensity” and “illuminance.” Luminous intensity is a photometric quantity measured in lumens per steradian (“lm/sr”), or candelas (“cd”). “Illuminance” is the total luminous flux incident per unit area on a surface. Illuminance is a measure of the density of the incident light on a surface.
It is now realized that there is a need to measure light intensity emitted by a source of light “in the field” rather then simply in a laboratory setting as is commonly done. There are few apparatus or methods of measuring light intensity in the field. Thus the inventors herein have undertaken the development of a compact, portable light-measuring meter that can measure light intensity in the field.
In many situations a homeowner or a municipal body may have a complaint of spillover lighting. Government regulations frequently limit the light intensity that can be emitted as light intensity correlates closely with the offending visual effects. Unfortunately, up to now, there was no way to measure the spillover or trespass light without placing a meter directly on an affected surface to measure illuminance on the surface. However placement of a light meter close to and proximate a surface is sometimes impractical or impossible. It would be very desirable if there were equipment available to measure the intensity emitted by a source of light of a particular source of light and not just illuminance on a surface lit by the source of light. The apparatus presented fulfills the goal of field measurement of light intensity by providing a compact, portable, light weight light measuring meter that can measure the intensity of light emitted from a distant source of light.
It has also been found that it is more useful to measure intensity of a source of light at a point distant from the source of light, rather than to measure the total illuminance of light falling on a surface.